Optimum reproduction of medical gray scale images requires an as large as possible number of brightness levels that can be distinguished by human observers. The number of shades of gray that can be distinguished in practice is dependent on the brightness that can be achieved and on the contrast range of the reproduction medium. Therefore, diagnostic applications usually employ film copies, since films have a contrast range of up to three orders of magnitude and are observed on view boxes that produce luminance values of up to 6000 cd/m2.
However, because of the deployment of contemporary digital imaging methods such as, for example X-ray computed tomography or magnetic resonance tomography as well as the use of image filing systems, there is an increasing tendency to reproduce digital image data also on monitors. Customary color monitors as they are encountered, for example in personal computers are often used for this purpose. In comparison with view boxes such monitors have a low light level with a luminance of typically 100 cd/m2 so that they produce significantly fewer shades of gray that can be differentiated.
Therefore, U.S. Pat. No. 3,541,233 and FR 2 301 021 deal with attempts to compensate the smaller reproduction range of color monitors by means of false color reproduction. According to this false color reproduction method a shade of gray is represented by an arbitrary color that is selected at random. The originally monochrome images, therefore, have a colored appearance. Because the human eye is capable of differentiating a number of colors that is much larger than the number of shades of gray that can be differentiated, the false color reproduction method also enables differentiation by the human observer of those shades of gray that could no longer be differentiated on the basis of their brightness value. The false color reproduction method, however, is inadequate for the reproduction of medical gray scale images, because the optical density of the monochrome images reflects properties of the tissue and hence is of essential importance in the image interpretation by the physician. This density information is arbitrarily encoded in images that are reproduced by the false color method, thus preventing interpretation.
Considering the foregoing it was an object of the present invention to provide a method of reproducing a gray scale image in colors which enables a large number of shades of gray to be reproduced so that they can be differentiated and at the same time enables the interpretation of medical gray scale images.